feat(documents): add metadataComplete flag and enrichment queue endpoints

Adds a metadata_complete column (default true for existing rows) to drive
the enrichment queue. New drop-zone uploads always start as false; createDocument
uses an explicit DTO flag or a heuristic (any of date/sender/receivers present →
true); the mass importer applies the same heuristic per row.

New endpoints: GET /api/documents/incomplete-count, /incomplete, /incomplete/next.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V15__add_metadata_complete_to_documents.sql

@@ -0,0 +1,6 @@
+-- Add metadata_complete flag to documents.
+-- Existing rows default to true (already reviewed before this feature existed).
+-- New documents created via Java will receive false from the entity default.
+
+ALTER TABLE documents
+    ADD COLUMN metadata_complete BOOLEAN NOT NULL DEFAULT TRUE;